1. Field of the Invention
The present invention is based on devices for measuring at least one parameter of a streaming fluid medium, in particular a fluid medium streaming through a flow pipe, as it is known in various fields of technology.
2. Description of Related Art
In many processes such as, for instance, in the field of industrial processing engineering, chemistry or machine construction, fluid media, especially masses of gas (e.g. an air mass) having particular properties (such as temperature, pressure, flow velocity, mass flow, etc.) have to be supplied in a defined manner. Among these are combustion processes, in particular, which run under regulated conditions.
An important application example is the combustion of fuel in internal combustion engines of motor vehicles, especially ones having subsequent catalytic exhaust gas purification, in which a certain air mass has to be supplied per unit time in a controlled manner (air mass flow). Various types of sensors are used to measure the air-mass throughput. One sensor type known from the related art is what is generally known as a hot-film air mass sensor (HFM), which is described in one specific embodiment in published German patent document DE 196 01 791, for example. A sensor chip, which has a thin sensor diaphragm, e.g., a silicon sensor chip, is generally utilized in such hot-film air mass meters. At least one thermal resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically situated on the sensor diaphragm. An air flow that is routed across the diaphragm causes a change in the temperature distribution pattern, which in turn is detectable by the temperature measuring resistors and is able to be analyzed with the aid of a control and evaluation circuit. Thus, for instance, an air mass flow is able to be determined from a difference in resistance of the temperature measuring resistors. A number of other variations of this sensor type are known from the related art.
One problem with such a type of sensor known from published German patent document DE 101 11 840, for instance, is that contamination of the sensor element may frequently occur, such as contamination by water, oil or other fluids, or other types of soiling. As a rule, the sensor chip is used directly in the intake tract of the internal combustion engine or in a bypass to the intake tract of the internal combustion engine. During operation of the internal combustion engine, water or oil can deposit on the sensor chip and on the sensor diaphragm, in particular. This soiling deposit can lead to an undesired effect on the measuring signal of the sensor, especially because a fluid film on the surface of the sensor affects the thermal conductivity of the surface, which results in a falsification of the measuring signals. Similar problems occur not only in hot-film air mass meters, but in other types of sensors as well, which are used for measuring fluid parameters in a fluid flow.
In order to solve these problems with respect to contamination, and especially to prevent water and oil from reaching the sensor chip, various approaches are known from the related art. One approach, which is described in published German patent document DE 10 2004 022 271, for example, is the use of what is known as a bypass. In this connection, a plug-in sensor is utilized, which is plugged into an intake pipe via a plug part, an inlet opening being provided in the plug part on the inflow side. A main flow channel is provided in the plug-in sensor, through which a substantial part of the flow flows from the inlet opening to a discharge opening. At a sharp edge, which is also referred to as “nose”, a bypass channel branches off from the main channel, in which bypass a sensor chip is disposed. The bypass channel eventually discharges into an outlet opening at the underside of the plug-in sensor. The sharp edge at the branching point of the bypass channel has the effect that water droplets, because of their mass inertia, are able to penetrate into the bypass channel only with difficulty, and that they remain in the main channel for the most part.
However, in the plug-in sensor constructions described in the related art it is disadvantageous that the plug-in sensors described, because of their aerodynamically disadvantageous shape often cause problems in the intake tract with regard to a loss in pressure attributable to flow resistance. In addition, the signal reproducibility of the signals from such sensors is comparatively low. For this reason, published German patent document DE 10 2004 022 271 provides a construction in which a flow diversion part is fixedly installed as a separate component in the flow pipe. As an alternative, a design of the flow diversion part in one piece with the plug-in sensor is also provided. Furthermore, a flow-conducting wall is fixedly installed in the flow pipe, which is intended to calm the flow downstream from the plug-in sensor.
The construction known from published German patent document DE 10 2004 022 271, however, entails various disadvantages in practice. One disadvantage is, for instance, that the flow-diversion part is normally fixedly installed in a section of the flow pipe. This fixed installation entails additional costs in the manufacture of this pipe section, just as does the provision of the additional flow-conducting wall. Furthermore, in the development as one piece the installation length is also too long for many applications. In addition, the construction described in published German patent document DE 10 2004 022 271 is able to be further optimized with respect to the pressure drop at the plug-in sensor.